1. Field of the Invention
The present invention relates to a device for driving a solenoid used in a torque transmitting mechanism or the like for an electronically controlled four-wheel drive vehicle or the like, and also to a control device for an electromagnetic actuator including the solenoid.
2. Description of the Related Art
For example, a torque transmitting mechanism or the like for an electronically controlled four-wheel drive vehicle includes a pair of right and left planetary gear sets and a pair of brake mechanisms for variably controlling the respective sun gears connected to the pair of planetary gear sets. Each brake mechanism includes a wet multiplate brake and an electromagnetic actuator for operating the multiplate brake. The electromagnetic actuator is composed of a core (yoke) having an annular groove, a solenoid inserted in the annular groove of the core, an armature opposed to the core with a given gap defined therebetween, and a piston integrally connected to the armature.
When a current is passed through the solenoid to energize the solenoid, the armature is attracted to the core by the solenoid, thereby generating a thrust. As a result, the piston integrally connected to the armature is operated to engage the multiplate brake, thereby generating a brake torque. By controlling the currents passing through the right and left solenoids according to a turning direction and a steering force or steering angle, output torques to be transmitted to the right and left rear axles can be variably controlled.
Generally carried out as a control method for a solenoid drive current is the combination of current feedback control using a PID controller and current control by a pulse width modulation (PWM) duty signal using a switching element and a current circulating diode. In this control method, the on-duty value as an output from the PID controller has an effective range of 0% to 100%. By adjusting the proportional term constant, integral term constant, and derivative term constant of the PID controller to optimum values, a steady-state deviation of a drive current (actual current) from a target current is eliminated, and a suitable overshoot is generated upon rising of the drive current.
In a control system mounted on a vehicle, however, the power supply voltage for the solenoid is generally a single power supply voltage to be supplied from a battery or the like. Accordingly, the conventional current control by the PWM duty signal using the switching element and the current circulating diode has the following problem. That is, even though the on-duty value is reduced to 0% to turn off the duty output in the case that the target current is rapidly reduced, falling of the drive current is limited by a time constant determined by the inductance component and the resistance component of the solenoid, causing a deterioration in response characteristic.
Further, the conventional current feedback control using the PID controller has the following problem. That is, rising of the actual current delays from the target current, causing a deterioration in response characteristic of the drive torque to result in defective behavior of the vehicle. Further, the brake (clutch) plates and the brake (clutch) discs configuring the wet multiplate brake (clutch) are deteriorated with secular change (wearing), and the initial gap between the core and the armature in the off state of the solenoid is accordingly reduced. Such a reduction in the initial gap causes an increase in the inductance component of the solenoid, thus degrading the response characteristic upon rising of the drive current.